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Better safeguards for sensitive information (Vol. 50, No. 2)
Study improves the lower boundary and secret key capacity of an encryption channel
The secure encryption of information units based on a method called quantum key distribution (QKD) involves distributing secret keys between two parties—namely, Alice, the sender, and Bob, the receiver—by using quantum systems as information carriers. However, the most advanced quantum technology, QKD, is currently limited by the channel's capacity to send or share secret bits. In a recent study the authors show how to better approach the secret key capacity by improving the channel's lower boundary. They focus on a particular type of channel, called the noisy thermal amplifier channel, where the input signals are amplified together with noise induced by the thermal environment. The authors calculate the highest-known amount of secret information units, or bits, that Alice and Bob can share via such a channel. This is done by injecting controlled noise—made up of well-defined thermal agitation—into the detection apparatuses. By optimising over this noise, they improve the lower boundary of the capacity in the amplifier channel. The authors also confirm that the distribution of secret keys over this channel may occur at higher rates than the transmission of quantum information itself.
G. Wang, C. Ottaviani, H. Guo, and S. Pirandola, Improving the lower bound to the secret-key capacity of the thermal amplifier channel, Eur. Phys. J. D 73, 17 (2019)
[Abstract]
Spin freezing and the Sachdev-Ye model (Vol. 50, No. 2)
The infinite-range, random-exchange Heisenberg spin model introduced in 1993 by Sachdev and Ye describes a non-Fermi liquid metal without quasi-particles, which resembles the bad-metal state of unconventional superconductors. Because of the somewhat artificial nature of the model, it is however not obvious how to connect this result to phenomena observed in strongly correlated materials. The latter are typically described by the Hubbard model and its multi-orbital extensions. Interestingly, the same non-Fermi liquid exponents as in the Sachdev-Ye model are generically observed in the correlated metallic phase of multi-orbital Hubbard models with Hund coupling. Our analysis suggests that the Sachdev-Ye model can be regarded as an effective description of a spin-freezing crossover regime with fluctuating local moments and that the variance of the random coupling in the Sachdev-Ye model is related to the Hund coupling. This analogy provides new insights into the nature of non-Fermi liquid metals, and into the close connection between spin freezing and unconventional superconductivity.
Ph. Werner, A.J. Kim and Sh. Hoshino, Spin freezing and the Sachdev-Ye model, EPL 124, 57002 (2018)
[Abstract]
Multimodal microscope enables structural and functional cellular imaging (Vol. 50, No. 2)
Answering cell physiology and pharmacology research questions often requires structural and functional information to be obtained from a network of cells. The authors have developed a multi-modal imaging system based on surface plasmon resonance (SPR) that combines several additional imaging modalities including bright-field, epifluorescence, total internal reflection microscopy and SPR fluorescence microscopy. The microscope features a wide field of view that can study ~40 cells simultaneously with subcellular resolution.
SPR is the collective oscillation of free electrons in a metal excited by polarized light. The resonance condition is highly dependent upon the refractive index of the media. Exploiting this allows the detection of both spatial and temporal variations in refractive index (RI) label-free.
In this work the authors describe a detailed design of the microscopy platform including standard tests for characterization of spatial resolution and sensitivity. Using SPR for imaging requires that the cell of interest is closely adhered to the surface. The spatial variation of refractive index was shown to be reasonably homogenous from a cultured neuron. Finally, a prototypical functional imaging experiment is reported where spatiotemporal cellular functions of stem cell-derived cardiomyocytes have been realised by detecting localized contractions.
C. L. Howe, K. F. Webb, S.A. Abayzeed, D. J. Anderson, C. Denning and N. A. Russell, Surface plasmon resonance imaging of excitable cells, J. Phys. D: Appl. Phys. 52, 104001 (2019)
[Abstract]
Lattice Improvement in Lattice Effective Field Theory (Vol. 50, No. 2)
Lattice calculations using the framework of effective field theory have been applied to a wide range of few-body and many-body systems. One of the challenges of these calculations is to remove systematic errors arising from the nonzero lattice spacing. While the lattice improvement program pioneered by Symanzik provides a formalism for doing this and has already been utilized in lattice effective field theory calculations, the effectiveness of the improvement program has not been systematically benchmarked.
In this work lattice improvement is used to remove lattice errors for a one-dimensional system of bosons with zero-range interactions. To this aim the improved lattice action up to next-to-next-to-leading order is constructed and it is verified that the remaining errors scale as the fourth power of the lattice spacing for observables involving as many as five particles. These results provide a guide for increasing the accuracy of future calculations in lattice effective field theory with improved lattice actions.
N. Klein, D. Lee and U.-G. Meißner,, Lattice improvement in lattice effective field theory, Eur. Phys. J. A 54, 233 (2018)
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